Typical pneumatic nebulizers, such as the Meinhard TR 30-C3 nebulizer, operate at liquid sample flow rates of about 500 .mu.l/min or greater. The Meinhard nebulizer consists of a rigid inner glass capillary tube drawn to a fine tip, surrounded by another glass tube drawn concentrically to a conical tip. The nebulizer operates through the interaction of a liquid stream in the inner capillary and a gas stream in the annular space-between the capillary tubes causing droplet formation. The Meinhard nebulizer suffers from a number of drawbacks, including that it tends to block up due to its converging tip. Once blocked, it is usually discarded.
Nebulizers which employ parallel coaxial tubes tend to avoid blockage problems. One such nebulizer is that of the application GB 2 203 241 to Willoughby et al. In this nebulizer velocity of the entraining gas combined with thermally induced solvent evaporation serves to cause a breakup of the liquid sample jet into liquid particles to produce an aerosol. This nebulizer is described to operate over liquid sample flow rates from 10-2000 .mu.l/min. However, such nebulizers are not known to work well at low liquid flow rates or when the end of the inner capillary tube extends out beyond the end of the outer capillary tube. By low liquid flow rates, we mean 500 .mu.l/min or less. This nebulizer is not described to cause an oscillation of the capillary tube by creating instability in the system, but rather describes that the aerosol is created by the combination of the entraining gas velocity and liquid sample heating. Another example of a known nebulizer which incorporates a coaxial tube arrangement is disclosed in U.S. Pat. No. 4,924,097 to Browner et al.
Another form of such a nebulizer is the direct injection nebulizer (DIN) of Wiederin et al. for inductively coupled plasma mass spectrometry (ICP/MS). Anal. Chem., 63, 219-225 (1991). Wiederin et al. discloses a DIN assembly consisting of a length of fused silica capillary tubing having a 50 .mu.m inner diameter and a 200 .mu.m outer diameter disposed within a stainless steel tube serving as the nebulizer. The stainless steel tube has a 250 .mu.m inner diameter and a 1.6 .mu.m outer diameter. Thus, a 25 .mu.m annular space is provided between the stainless steel tube and the fused silica capillary tubing. The inner tubing is positioned to extend approximately 100 .mu.m beyond the end of the stainless steel nebulizer tube. The DIN assembly is positioned within the converging end of the quartz injector tube of the torch for injecting sample directly into the plasma of the ICP/MS. The liquid sample flow rate was optimized at 120 .mu.l/min. with a corresponding gas nebulizer gas pressure of 200 psi and a nebulizer gas flow rate of 1.0 .mu.L/min. In operation, Wiederin et al. observed a slight hissing sound, like most pneumatic nebulzers, which became quite loud when the plasma was started and liquids were nebulized. Wiederin et al. comments that the precision of their nebulizer was notably poorer when positioned in a spray chamber similar to a conventional pneumatic nebulizer.
The operation of the Wiederin et al. DIN assembly has been categorized by Shum et al. See Appl. Spectrosc. 47, 575, (1993). When the Wiederin et al. DIN assembly was operated at a liquid flow rate of 100 .mu.L/min., it was found that the inclusion of methanol as an organic modifier to a liquid water sample had a dramatic effect on the size of the aerosol droplet distribution attained, as illustrated by Shum et al. It was also observed that varying nebulizer gas flow rates from 0.3-0.9 L/min., while maintaining the liquid sample flow rate constant, had little effect on the size of aerosol droplets obtained.
It is also known in the prior art to utilize ultrasonic transducers to break up a liquid sample jet into liquid droplets. For example, U.S. Pat. No. 4,112,297, to Miyagi et al., discloses a nebulizer which includes an ultrasonic transducer used to create the particle beam. U.S. Pat. No. 4,403,147, to Melera et al., incorporates an acoustic transducer, such as a piezoelectric transducer which may be used to stimulate the probe to break up the liquid stream. An example of a nebulizer which employs an oscillating piezoelectric ceramic transducer is disclosed in U.S. Pat. No. 3,790,079 to Berglund. In such nebulizers, which operate on the basis of a transducer, the frequency of operation effects the aerosol droplet size. They also are much more expensive than a coaxial tube nebulizer.
U.S. Pat. No. 3,108,749, to Draver et al., and Reissue patent RE.25,744, to Drayer et al., are representative of other forms of pressurized air induced vibrating atomizers.
None of the above described nebulizers or atomizers are known to operate reliably at microflow liquid flow rates. By microflow liquid flow rates, we mean 100 .mu.l/min or less and preferably below 30 .mu.l/min. Conventional nebulizers typically operate at liquid flow rates greater than 500 .mu.l/min. However, at such liquid flow rates the solvent delivery rate to any mass spectrometer or plasma source detector will be so great as to cause considerable source instability. Hence, a solvent removal step, through either a droplet removal chamber or a two-(or three-)stage pressure reduction skinmer device is necessary. With benchtop liquid chromatography/mass spectrometry systems (LC/MS), the relatively low pumping capacity of the source makes coupling with high flow nebulizers impractical. At liquid flow rates of about 500 .mu.l/min or less, the conventional nebulizer becomes unsatisfactory and unreliable. The lowest liquid flow rate reported by Wiederin et al. for their direct injection nebulizer is 30 .mu.l/min. However, they teach away from such lower flows by teaching that the liquid flow rate was optimized at 120 .mu.l/min.
It is important to recognize the relative definitions describing the range of velocities of the liquid flow. In inductively coupled plasma (ICP) work, conventional nebulizers do well in relatively high liquid flow rates (defined here as 1-2 ml/min), but perform poorly in relatively low liquid flow rates (defined here as &lt;100 .mu.l/min). Conversely, the prior art oscillating capillary nebulizers (OCN), perform well at low liquid flow rates compared to other devices in the fields of ICP/AES and ICP/MS.
In reference to electrospray techniques, conventional sources use only high voltage to produce the aerosols, described below. Therefore, the sources can only handle a very low flow (i.e., microflow) rate and a very low range of flow rates (1-15 .mu.l/min). Accordingly, it will be understood by one skilled in the art that the descriptions "low" and "high" have different meanings relative to the different techniques (ICP or electrospray).
It is known in the prior art to utilize an electrospray ionization technique which incorporates a fine capillary tube made of conducting metal attached to a high voltage source. In this technique, liquid is directed through the capillary tube in which the end is connected to one pole of a high voltage source. The end of the capillary tube is spaced from the orifice plate through which ions travel into the mass analyzer vacuum chamber. The capillary is connected to the pole of the high voltage source. The electric field generates charged droplets, and the droplets evaporate to produce ions. Examples of such a technique are disclosed in U.S. Pat. No. 4,209,696 to Fite, and U.S. Pat. No. 4,861,988 to Henion et al.
In ICP/AES and ICP/MS, an OCN is used to produce and control the aerosols from the liquid. The aerosols then enter the plasma source and get excited/ionized, and finally, signals are generated by measuring the photons (AES) or ions (MS). In conventional electrospray, a high voltage is applied on the capillary. The high voltage produces charged aerosols, which then can be directly analyzed using a mass spectrometer. Therefore, electrospray is an ionization method, and may be used as an ion source.
Yet, the conventional electrospray method has several disadvantages. Firstly, it can handle only a very small flow, typically only up to about 15 .mu.l/min. Faster pumping produces larger droplets, thus causing the ion signal to fall off and also to become unstable. Secondly, the high voltages needed to disperse a larger liquid flow into fine droplets tend to create an electrical or corona discharge. The discharge adds complexity to the spectrum produced by the mass analyzer, causing difficulties in interpretation, and in addition, for unknown reasons, it tends to suppress the ion signals from the evaporated droplets. Other disadvantages are that dynamic range is usually less than two orders of magnitude, and that this process does not work well with additives. In addition, the electrospray method requires that the proportion of water in liquid be low, since otherwise a stream of large droplets tends to be produced. The large droplets reduce the sensitivity and also affects the stability of the ion signal, i.e. large fluctuations occur in the ion signal. Also, the prior art devices and methods tend to fail in performance and reliability as higher concentrations of water are used.
Therefore, a need exists in the art for a nebulizer which is capable of producing an aerosol at microflow liquid flow rates for employment with microflow chromatographic techniques and for use with bench top LC/MS, ICP/AES and ICP/MS instruments and which is capable of satisfactorily controlling the particle size and particle size distribution of the aerosol over wide concentration ranges of solvents and liquid flow rates, wider than those achievable with conventional electrospray methods. Accordingly, the present invention employs an inner/outer coaxial tube arrangement utilizing the electrospray technique which can meet these needs and overcome the above deficiencies.